Z-buffering:
Z-buffering is a hardware routine that selects
which polygons to draw and which to omit. Rasterizing
polygons on-screen takes up a lot of time, it is therefore important that the
CPU time is not taken up drawing polygons that
cannot be seen by the user (i.e. they are off-screen or hidden behind other
polygons that are either moving or closer). This routine speeds up the process
of drawing polygons on-screen and permits a faster refresh rate.